Why does array[idx++]+=”a” increase idx once in Java 8 but twice in Java 9 and 10?

For a challenge, a fellow code golfer wrote the following code:

import java.util.*;
public class Main {
  public static void main(String[] args) {
    int size = 3;
    String[] array = new String[size];
    Arrays.fill(array, "");
    for (int i = 0; i <= 100;) {
      array[i++ % size] += i + " ";
    }
    for (String element: array) {
      System.out.println(element);
    }
  }
}

When running this code in Java 8, we get the following result:

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 97 100 
2 5 8 11 14 17 20 23 26 29 32 35 38 41 44 47 50 53 56 59 62 65 68 71 74 77 80 83 86 89 92 95 98 101 
3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99 

When running this code in Java 10, we get the following result:

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102 
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 

The numbering is entirely off using Java 10. So what is happening here? Is it a bug in Java 10?

Follow ups from the comments:

  • The issue appears when compiled with Java 9 or later (we found it in Java 10). Compiling this code on Java 8, then running in Java 9 or any later version, including Java 11 early access, gives the expected result.

  • This kind of code is non-standard, but is valid according to the spec. It was found by Kevin Cruijssen in a discussion in a golfing challenge, hence the weird use case encountered.

  • Didier L simplified the issue with this much smaller and more understandable code:

      class Main {
        public static void main(String[] args) {
          String[] array = { "" };
          array[test()] += "a";
        }
        static int test() {
          System.out.println("evaluated");
          return 0;
        }
      }
    

    Result when compiled in Java 8:

      evaluated
    

    Result when compiled in Java 9 and 10:

      evaluated
      evaluated
    
  • The issue seems to be limited to the string concatenation and assignment operator (+=) with an expression with side effect(s) as the left operand, like in array[test()]+="a", array[ix++]+="a", test()[index]+="a", or test().field+="a". To enable string concatenation, at least one of the sides must have type String. Trying to reproduce this on other types or constructs failed.

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